The present invention relates to a method for producing an extrusion die which is employed to extrude a ceramic honeycomb structure for use as a catalyst carrier or support for purifying exhaust gases of an automobile or the like.
One example of the above described extrusion die is disclosed in U.S. Pat. No. 4,373,895 and comprises a plurality of discrete feed passages which are drilled from one end surface of a die body towards the other end surface thereof separately, interconnected grid-shaped extrusion slots which are formed from the other end surface of the die body and interconnected grid-shaped pooling slots which are positioned between the feed passages and the extrusion slots so as to be interconnected therewith and have a width larger than that of the extrusion slots.
In the above extrusion die, an extrudable material such as ceramic slurry is supplied through the feed passages, termporarily pooled in the pooling slots and then is fed into the extrusion slots. The extrudable material is successively extruded by the extrusion slots to form a ceramic honeycomb structure.
There is disclosed in the above described United States Patent, a method for producing an extrusion die, which comprises the steps of preparing two metallic blocks, forming feed passages from one end surface of a thick plate-shaped first metallic block, forming pooling slots from the other end surface thereof, joining a thin plate-shaped second block to the other end surface of the thick plate-shaped first metallic block by brazing, welding or the like, and forming extrusion slots from the outer end surface of the second metallic block therethrough.
According to this method, the pooling slots can be formed by machining the end surface of the first metallic block so that dimensional accuracy can be improved. However, this method has a problem. Namely, the pooling slots are apt to be deformed due to heat treatment applied when the two metallic blocks are joined to each other. In this case, even if the extrusion slots are formed at predetermined distances on the second metallic block, the extrusion slots do not overlap the pooling slots accurately and they are not completely interconnected with each other. As a result, slurry is partially blocked from smoothly flowing from the pooling slots to the extrusion slots and the thickness of the obtained honeycomb structure becomes non-uniform.